Device-to-device (D2D) link adaptation

ABSTRACT

Channel quality may be measured in a device-to-device (D2D) communication network. The D2D communication network may include one or more D2D wireless transmit/receive units (WTRUs), wherein the D2D WTRUs may communicate using a D2D bandwidth. A D2D WTRU may receive a channel measurement resource configuration corresponding to a channel measurement resource. The D2D WTRU may further receive an RS on the channel measurement resource. The D2D WTRU may measure one or more channel state parameters from the channel measurement resource for a part of bandwidth overlapping with a D2D communication bandwidth, when the RS bandwidth is greater than the D2D communication bandwidth. The D2D WTRU may report the channel state parameters to a controlling entity. The controlling entity may configure a D2D frequency allocation between a transmitting device and a receiving device. The D2D frequency allocation may be based on the time averaged measurement.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Non-Provisional patentapplication No. 13/905,885, filed May 30, 2013, which claims the benefitof U.S. Provisional Patent Application No. 61/653,523, filed May 31,2012, and U.S. Provisional Patent Application No. 61/798,356, filed Mar.15, 2013, the contents of which are hereby incorporated by referenceherein.

BACKGROUND

The recent rise in popularity of smart phones is changing the way peopleuse their wireless devices and how services are offered. For example,location based services are becoming more and more the norm. Likewise,social media applications employing location information are emergingand are expected to grow in use in the near future. It is expected thatapplications and services involving two or more nearby devices may causelarge amount of data traffic in networks. One approach to mitigate theincrease in network traffic due to these “Proximity Services” may be tolimit the traffic to the cell, for example allowing direct UserEquipment (UE)-to-UE and/or device-to-device (D2D) communications and/orto have the eNode-B (eNB) relay the information. This last optionreduces the data traffic on the network as the data packets may nolonger be carried to the SGSN and back to the originating eNB, therebyoffloading the network. This local offload approach may be used whenlarge amount of data is to be exchanged.

The spectacular increase in wireless data services in recent years canbe partially attributed to the improvement in radio communicationstechnology. More particularly, the use of link-adaptation has had alarge impact on the spectrum efficiency and the improvement in theoffered data rates. In mobile wireless communications, the propagationchannel experiences fades which varies in time due to the devicemobility. Due to this fast fading, the quality of the channel varies andlikewise the amount of data that can be carried reliably varies in time.Fast link adaptation allows the transmitter to adapt the amount of dataas a function of the channel quality thereby improving the overallspectrum efficiency.

At a high level, link adaptation between, for example, a user's handsetand an eNB may entail the transmitter and/or the scheduler determiningthe characteristics of the propagation channel (also referred to aschannel state information or CSI). In practice, this may be implementedvia the receiver sending CSI feedback to the transmitter; althoughsometimes this CSI can be inferred, at least partially, in differentways (e.g. via reciprocity in the case of TDD). In traditional systemssuch as LTE/LTE-A, HSPA, 802.11, 802.16 and others, the receivermeasures the propagation channel and sends the CSI back to thetransmitter (in the case of the uplink) so that the scheduling can takeadvantage of the channel conditions.

SUMMARY

Channel quality may be measured in a device-to-device (D2D)communication network. The D2D communication network may include one ormore D2D wireless transmit/receive units (WTRUs), wherein the D2D WTRUsmay communicate using a D2D bandwidth. A D2D WTRU may receive a channelmeasurement resource configuration (e.g., via a radio resource control(RRC) configuration signal) corresponding to a channel measurementresource. The channel measurement resource may include one or more of asubframe, a slot, a resource block (RB), a physical resource block(PRB), or a resource element (RE). The channel measurement resourceconfiguration may further include a reference signal (RS) bandwidth. TheD2D WTRU may further receive an RS on the channel measurement resource.The RS may include one or more of a sounding reference signal (SRS), auplink demodulation reference signal (UL DM-RS), a channel stateinformation reference signal (CSI-RS), a downlink demodulation referencesignal (DL DM-RS), a discovery signal, a preamble or a postamble signal.The RS may be a periodic signal or an aperiodic signal.

The D2D WTRU may measure one or more channel state parameters from thechannel measurement resource for a part of bandwidth overlapping with aD2D communication bandwidth, when the RS bandwidth is greater than theD2D communication bandwidth. The measuring device may exclude a portionof the RS bandwidth that does not overlap with the D2D communicationbandwidth from measurement. The measuring device may limit measurementof the one or more channel state parameters to a portion of the RSbandwidth that may overlap with the D2D bandwidth. The measuring devicemay determine a measurement bandwidth. The measurement bandwidth mayinclude the portion of the overlap between the RS bandwidth and the D2Dbandwidth. The D2D WTRU or measuring device may report the channel stateparameters to a controlling entity. The D2D WTRU or measuring device maymeasure the channel state parameters instantaneously.

The controlling entity may be a transmitting device, a receiving device,or an eNB. The D2D WTRU may measure the channel state parameters includea metric indicative of a channel quality, e.g., channel stateinformation (CSI). The D2D WTRU may measure the channel state parameterson a time average basis and report a time averaged measurement to thecontrolling entity.

The controlling entity may configure a D2D frequency allocation betweena transmitting device and a receiving device. The D2D frequencyallocation may be based on the time averaged measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding may be had from the following description,given by way of example in conjunction with the accompanying drawingswherein:

FIG. 1A is a system diagram of an example communications system in whichone or more disclosed embodiments may be implemented;

FIG. 1B is a system diagram of an example wireless transmit/receive unit(WTRU) that may be used within the communications system illustrated inFIG. 1A;

FIG. 1C is a system diagram of an example radio access network and anexample core network that may be used within the communications systemillustrated in FIG. 1A;

FIG. 1D is a system diagram of another example radio access network andan example core network that may be used within the communicationssystem illustrated in FIG. 1A; and

FIG. 1E is a system diagram of another example radio access network andan example core network that may be used within the communicationssystem illustrated in FIG. 1A.

FIG. 2 is a diagram illustrating an example of various device including,e.g., a transmitting device, a receiving device, a measuring device,and/or a controlling entity.

FIG. 3 is a diagram illustrating an example scenario of FIG. 2, wherethe receiving device may be the measuring device.

FIG. 4 is a diagram illustrating an exemplary reporting of CSI for thesubcarriers overlapping with the allocated D2D communication bandwidth.

FIG. 5 is a diagram illustrating an example for transmitting anaperiodic reference signal.

FIG. 6 is a diagram illustrating an example of the control information(e.g., device to device control information (D2D-CI)) that may betransmitted in the OFDM symbol preceding a DM-RS symbol, e.g., on anUL-like subframe.

FIG. 7 is a diagram illustrating an example of the control information(e.g., D2D-CI) that may be multiplexed with the first DM-RS symbol,e.g., on an UL-like subframe.

DETAILED DESCRIPTION

A detailed description of illustrative embodiments will now be describedwith reference to the various figures. Although this descriptionprovides a detailed example of possible implementations, it should benoted that the details are intended to be exemplary and in no way limitthe scope of the application.

FIG. 1A is a diagram of an example communications system 100 in whichone or more disclosed embodiments may be implemented. The communicationssystem 100 may be a multiple access system that provides content, suchas voice, data, video, messaging, broadcast, etc., to multiple wirelessusers. The communications system 100 may enable multiple wireless usersto access such content through the sharing of system resources,including wireless bandwidth. For example, the communications systems100 may employ one or more channel access methods, such as code divisionmultiple access (CDMA), time division multiple access (TDMA), frequencydivision multiple access (FDMA), orthogonal FDMA (OFDMA), single-carrierFDMA (SC-FDMA), and the like.

As shown in FIG. 1A, the communications system 100 may include wirelesstransmit/receive units (WTRUs) 102 a, 102 b, 102 c, 102 d, a radioaccess network (RAN) 104, a core network 106, a public switchedtelephone network (PSTN) 108, the Internet 110, and other networks 112,though it will be appreciated that the disclosed embodiments contemplateany number of WTRUs, base stations, networks, and/or network elements.Each of the WTRUs 102 a, 102 b, 102 c, 102 d may be any type of deviceconfigured to operate and/or communicate in a wireless environment. Byway of example, the WTRUs 102 a, 102 b, 102 c, 102 d may be configuredto transmit and/or receive wireless signals and may include userequipment (UE), a mobile station, a fixed or mobile subscriber unit, apager, a cellular telephone, a personal digital assistant (PDA), asmartphone, a laptop, a netbook, a personal computer, a wireless sensor,consumer electronics, and the like.

The communications systems 100 may also include a base station 114 a anda base station 114 b. Each of the base stations 114 a, 114 b may be anytype of device configured to wirelessly interface with at least one ofthe WTRUs 102 a, 102 b, 102 c, 102 d to facilitate access to one or morecommunication networks, such as the core network 106, the Internet 110,and/or the networks 112. By way of example, the base stations 114 a, 114b may be a base transceiver station (BTS), a Node-B, an eNode B, a HomeNode B, a Home eNode B, a site controller, an access point (AP), awireless router, and the like. While the base stations 114 a, 114 b areeach depicted as a single element, it will be appreciated that the basestations 114 a, 114 b may include any number of interconnected basestations and/or network elements.

The base station 114 a may be part of the RAN 104, which may alsoinclude other base stations and/or network elements (not shown), such asa base station controller (BSC), a radio network controller (RNC), relaynodes, etc. The base station 114 a and/or the base station 114 b may beconfigured to transmit and/or receive wireless signals within aparticular geographic region, which may be referred to as a cell (notshown). The cell may further be divided into cell sectors. For example,the cell associated with the base station 114 a may be divided intothree sectors. Thus, in one embodiment, the base station 114 a mayinclude three transceivers, i.e., one for each sector of the cell. Inanother embodiment, the base station 114 a may employ multiple-inputmultiple output (MIMO) technology and, therefore, may utilize multipletransceivers for each sector of the cell.

The base stations 114 a, 114 b may communicate with one or more of theWTRUs 102 a, 102 b, 102 c, 102 d over an air interface 116, which may beany suitable wireless communication link (e.g., radio frequency (RF),microwave, infrared (IR), ultraviolet (UV), visible light, etc.). Theair interface 116 may be established using any suitable radio accesstechnology (RAT).

More specifically, as noted above, the communications system 100 may bea multiple access system and may employ one or more channel accessschemes, such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA, and the like. Forexample, the base station 114 a in the RAN 104 and the WTRUs 102 a, 102b, 102 c may implement a radio technology such as Universal MobileTelecommunications System (UMTS) Terrestrial Radio Access (UTRA), whichmay establish the air interface 116 using wideband CDMA (WCDMA). WCDMAmay include communication protocols such as High-Speed Packet Access(HSPA) and/or Evolved HSPA (HSPA+). HSPA may include High-Speed DownlinkPacket Access (HSDPA) and/or High-Speed Uplink Packet Access (HSUPA).

In another embodiment, the base station 114 a and the WTRUs 102 a, 102b, 102 c may implement a radio technology such as Evolved UMTSTerrestrial Radio Access (E-UTRA), which may establish the air interface116 using Long Term Evolution (LTE) and/or LTE-Advanced (LTE-A).

In other embodiments, the base station 114 a and the WTRUs 102 a, 102 b,102 c may implement radio technologies such as IEEE 802.16 (i.e.,Worldwide Interoperability for Microwave Access (WiMAX)), CDMA2000,CDMA2000 1×, CDMA2000 EV-DO, Interim Standard 2000 (IS-2000), InterimStandard 95 (IS-95), Interim Standard 856 (IS-856), Global System forMobile communications (GSM), Enhanced Data rates for GSM Evolution(EDGE), GSM EDGE (GERAN), and the like.

The base station 114 b in FIG. 1A may be a wireless router, Home Node B,Home eNode B, or access point, for example, and may utilize any suitableRAT for facilitating wireless connectivity in a localized area, such asa place of business, a home, a vehicle, a campus, and the like. In oneembodiment, the base station 114 b and the WTRUs 102 c, 102 d mayimplement a radio technology such as IEEE 802.11 to establish a wirelesslocal area network (WLAN). In another embodiment, the base station 114 band the WTRUs 102 c, 102 d may implement a radio technology such as IEEE802.15 to establish a wireless personal area network (WPAN). In yetanother embodiment, the base station 114 b and the WTRUs 102 c, 102 dmay utilize a cellular-based RAT (e.g., WCDMA, CDMA2000, GSM, LTE,LTE-A, etc.) to establish a picocell or femtocell. As shown in FIG. 1A,the base station 114 b may have a direct connection to the Internet 110.Thus, the base station 114 b may not be required to access the Internet110 via the core network 106.

The RAN 104 may be in communication with the core network 106, which maybe any type of network configured to provide voice, data, applications,and/or voice over internet protocol (VoIP) services to one or more ofthe WTRUs 102 a, 102 b, 102 c, 102 d. For example, the core network 106may provide call control, billing services, mobile location-basedservices, pre-paid calling, Internet connectivity, video distribution,etc., and/or perform high-level security functions, such as userauthentication. Although not shown in FIG. 1A, it will be appreciatedthat the RAN 104 and/or the core network 106 may be in direct orindirect communication with other RANs that employ the same RAT as theRAN 104 or a different RAT. For example, in addition to being connectedto the RAN 104, which may be utilizing an E-UTRA radio technology, thecore network 106 may also be in communication with another RAN (notshown) employing a GSM radio technology.

The core network 106 may also serve as a gateway for the WTRUs 102 a,102 b, 102 c, 102 d to access the PSTN 108, the Internet 110, and/orother networks 112. The PSTN 108 may include circuit-switched telephonenetworks that provide plain old telephone service (POTS). The Internet110 may include a global system of interconnected computer networks anddevices that use common communication protocols, such as thetransmission control protocol (TCP), user datagram protocol (UDP) andthe internet protocol (IP) in the TCP/IP internet protocol suite. Thenetworks 112 may include wired or wireless communications networks ownedand/or operated by other service providers. For example, the networks112 may include another core network connected to one or more RANs,which may employ the same RAT as the RAN 104 or a different RAT.

Some or all of the WTRUs 102 a, 102 b, 102 c, 102 d in thecommunications system 100 may include multi-mode capabilities, i.e., theWTRUs 102 a, 102 b, 102 c, 102 d may include multiple transceivers forcommunicating with different wireless networks over different wirelesslinks. For example, the WTRU 102 c shown in FIG. 1A may be configured tocommunicate with the base station 114 a, which may employ acellular-based radio technology, and with the base station 114 b, whichmay employ an IEEE 802 radio technology.

FIG. 1B is a system diagram of an example WTRU 102. As shown in FIG. 1B,the WTRU 102 may include a processor 118, a transceiver 120, atransmit/receive element 122, a speaker/microphone 124, a keypad 126, adisplay/touchpad 128, non-removable memory 130, removable memory 132, apower source 134, a global positioning system (GPS) chipset 136, andother peripherals 138. It will be appreciated that the WTRU 102 mayinclude any sub-combination of the foregoing elements while remainingconsistent with an embodiment. It is noted that the components,functions, and features described with respect to the WTRU 102 may alsobe similarly implemented in a base station.

The processor 118 may be a general purpose processor, a special purposeprocessor, a conventional processor, a digital signal processor (DSP), aplurality of microprocessors, one or more microprocessors in associationwith a DSP core, a controller, a microcontroller, Application SpecificIntegrated Circuits (ASICs), Field Programmable Gate Array (FPGAs)circuits, any other type of integrated circuit (IC), a state machine,and the like. The processor 118 may perform signal coding, dataprocessing, power control, input/output processing, and/or any otherfunctionality that enables the WTRU 102 to operate in a wirelessenvironment. The processor 118 may be coupled to the transceiver 120,which may be coupled to the transmit/receive element 122. While FIG. 1Bdepicts the processor 118 and the transceiver 120 as separatecomponents, it will be appreciated that the processor 118 and thetransceiver 120 may be integrated together in an electronic package orchip.

The transmit/receive element 122 may be configured to transmit signalsto, or receive signals from, a base station (e.g., the base station 114a) over the air interface 116. For example, in one embodiment, thetransmit/receive element 122 may be an antenna configured to transmitand/or receive RF signals. In another embodiment, the transmit/receiveelement 122 may be an emitter/detector configured to transmit and/orreceive IR, UV, or visible light signals, for example. In yet anotherembodiment, the transmit/receive element 122 may be configured totransmit and receive both RF and light signals. It will be appreciatedthat the transmit/receive element 122 may be configured to transmitand/or receive any combination of wireless signals.

In addition, although the transmit/receive element 122 is depicted inFIG. 1B as a single element, the WTRU 102 may include any number oftransmit/receive elements 122. More specifically, the WTRU 102 mayemploy MIMO technology. Thus, in one embodiment, the WTRU 102 mayinclude two or more transmit/receive elements 122 (e.g., multipleantennas) for transmitting and receiving wireless signals over the airinterface 116.

The transceiver 120 may be configured to modulate the signals that areto be transmitted by the transmit/receive element 122 and to demodulatethe signals that are received by the transmit/receive element 122. Asnoted above, the WTRU 102 may have multi-mode capabilities. Thus, thetransceiver 120 may include multiple transceivers for enabling the WTRU102 to communicate via multiple RATs, such as UTRA and IEEE 802.11, forexample.

The processor 118 of the WTRU 102 may be coupled to, and may receiveuser input data from, the speaker/microphone 124, the keypad 126, and/orthe display/touchpad 128 (e.g., a liquid crystal display (LCD) displayunit or organic light-emitting diode (OLED) display unit). The processor118 may also output user data to the speaker/microphone 124, the keypad126, and/or the display/touchpad 128. In addition, the processor 118 mayaccess information from, and store data in, any type of suitable memory,such as the non-removable memory 130 and/or the removable memory 132.The non-removable memory 130 may include random-access memory (RAM),read-only memory (ROM), a hard disk, or any other type of memory storagedevice. The removable memory 132 may include a subscriber identitymodule (SIM) card, a memory stick, a secure digital (SD) memory card,and the like. In other embodiments, the processor 118 may accessinformation from, and store data in, memory that is not physicallylocated on the WTRU 102, such as on a server or a home computer (notshown).

The processor 118 may receive power from the power source 134, and maybe configured to distribute and/or control the power to the othercomponents in the WTRU 102. The power source 134 may be any suitabledevice for powering the WTRU 102. For example, the power source 134 mayinclude one or more dry cell batteries (e.g., nickel-cadmium (NiCd),nickel-zinc (NiZn), nickel metal hydride (NiMH), lithium-ion (Li-ion),etc.), solar cells, fuel cells, and the like.

The processor 118 may also be coupled to the GPS chipset 136, which maybe configured to provide location information (e.g., longitude andlatitude) regarding the current location of the WTRU 102. In additionto, or in lieu of, the information from the GPS chipset 136, the WTRU102 may receive location information over the air interface 116 from abase station (e.g., base stations 114 a, 114 b) and/or determine itslocation based on the timing of the signals being received from two ormore nearby base stations. It will be appreciated that the WTRU 102 mayacquire location information by way of any suitablelocation-determination method while remaining consistent with anembodiment.

The processor 118 may further be coupled to other peripherals 138, whichmay include one or more software and/or hardware modules that provideadditional features, functionality and/or wired or wirelessconnectivity. For example, the peripherals 138 may include anaccelerometer, an e-compass, a satellite transceiver, a digital camera(for photographs or video), a universal serial bus (USB) port, avibration device, a television transceiver, a hands free headset, aBluetooth® module, a frequency modulated (FM) radio unit, a digitalmusic player, a media player, a video game player module, an Internetbrowser, and the like.

FIG. 1C is a system diagram of the RAN 104 and the core network 106according to an embodiment. As noted above, the RAN 104 may employ aUTRA radio technology to communicate with the WTRUs 102 a, 102 b, 102 cover the air interface 116. The RAN 104 may also be in communicationwith the core network 106. As shown in FIG. 1C, the RAN 104 may includeNode-Bs 140 a, 140 b, 140 c, which may each include one or moretransceivers for communicating with the WTRUs 102 a, 102 b, 102 c overthe air interface 116. The Node-Bs 140 a, 140 b, 140 c may each beassociated with a particular cell (not shown) within the RAN 104. TheRAN 104 may also include RNCs 142 a, 142 b. It will be appreciated thatthe RAN 104 may include any number of Node-Bs and RNCs while remainingconsistent with an embodiment.

As shown in FIG. 1C, the Node-Bs 140 a, 140 b may be in communicationwith the RNC 142 a. Additionally, the Node-B 140 c may be incommunication with the RNC 142 b. The Node-Bs 140 a, 140 b, 140 c maycommunicate with the respective RNCs 142 a, 142 b via an Iub interface.The RNCs 142 a, 142 b may be in communication with one another via anIur interface. Each of the RNCs 142 a, 142 b may be configured tocontrol the respective Node-Bs 140 a, 140 b, 140 c to which it isconnected. In addition, each of the RNCs 142 a, 142 b may be configuredto carry out or support other functionality, such as outer loop powercontrol, load control, admission control, packet scheduling, handovercontrol, macrodiversity, security functions, data encryption, and thelike.

The core network 106 shown in FIG. 1C may include a media gateway (MGW)144, a mobile switching center (MSC) 146, a serving GPRS support node(SGSN) 148, and/or a gateway GPRS support node (GGSN) 150. While each ofthe foregoing elements are depicted as part of the core network 106, itwill be appreciated that any one of these elements may be owned and/oroperated by an entity other than the core network operator.

The RNC 142 a in the RAN 104 may be connected to the MSC 146 in the corenetwork 106 via an IuCS interface. The MSC 146 may be connected to theMGW 144. The MSC 146 and the MGW 144 may provide the WTRUs 102 a, 102 b,102 c with access to circuit-switched networks, such as the PSTN 108, tofacilitate communications between the WTRUs 102 a, 102 b, 102 c andtraditional land-line communications devices.

The RNC 142 a in the RAN 104 may also be connected to the SGSN 148 inthe core network 106 via an IuPS interface. The SGSN 148 may beconnected to the GGSN 150. The SGSN 148 and the GGSN 150 may provide theWTRUs 102 a, 102 b, 102 c with access to packet-switched networks, suchas the Internet 110, to facilitate communications between and the WTRUs102 a, 102 b, 102 c and IP-enabled devices.

As noted above, the core network 106 may also be connected to thenetworks 112, which may include other wired or wireless networks thatare owned and/or operated by other service providers.

FIG. 1D is a system diagram of the RAN 104 and the core network 106according to an embodiment. As noted above, the RAN 104 may employ anE-UTRA radio technology to communicate with the WTRUs 102 a, 102 b, 102c over the air interface 116. The RAN 104 may also be in communicationwith the core network 106.

The RAN 104 may include eNode-Bs 140 a, 140 b, 140 c, though it will beappreciated that the RAN 104 may include any number of eNode-Bs whileremaining consistent with an embodiment. The eNode-Bs 140 a, 140 b, 140c may each include one or more transceivers for communicating with theWTRUs 102 a, 102 b, 102 c over the air interface 116. In one embodiment,the eNode-Bs 140 a, 140 b, 140 c may implement MIMO technology. Thus,the eNode-B 140 a, for example, may use multiple antennas to transmitwireless signals to, and receive wireless signals from, the WTRU 102 a.

Each of the eNode-Bs 140 a, 140 b, 140 c may be associated with aparticular cell (not shown) and may be configured to handle radioresource management decisions, handover decisions, scheduling of usersin the uplink and/or downlink, and the like. As shown in FIG. 1D, theeNode-Bs 140 a, 140 b, 140 c may communicate with one another over an X2interface.

The core network 106 shown in FIG. 1D may include a mobility managementgateway (MME) 142, a serving gateway 144, and a packet data network(PDN) gateway 146. While each of the foregoing elements are depicted aspart of the core network 106, it will be appreciated that any one ofthese elements may be owned and/or operated by an entity other than thecore network operator.

The MME 142 may be connected to each of the eNode-Bs 142 a, 142 b, 142 cin the RAN 104 via an S1 interface and may serve as a control node. Forexample, the MME 142 may be responsible for authenticating users of theWTRUs 102 a, 102 b, 102 c, bearer activation/deactivation, selecting aparticular serving gateway during an initial attach of the WTRUs 102 a,102 b, 102 c, and the like. The MME 142 may also provide a control planefunction for switching between the RAN 104 and other RANs (not shown)that employ other radio technologies, such as GSM or WCDMA.

The serving gateway 144 may be connected to each of the eNode Bs 140 a,140 b, 140 c in the RAN 104 via the S1 interface. The serving gateway144 may generally route and forward user data packets to/from the WTRUs102 a, 102 b, 102 c. The serving gateway 144 may also perform otherfunctions, such as anchoring user planes during inter-eNode B handovers,triggering paging when downlink data is available for the WTRUs 102 a,102 b, 102 c, managing and storing contexts of the WTRUs 102 a, 102 b,102 c, and the like.

The serving gateway 144 may also be connected to the PDN gateway 146,which may provide the WTRUs 102 a, 102 b, 102 c with access topacket-switched networks, such as the Internet 110, to facilitatecommunications between the WTRUs 102 a, 102 b, 102 c and IP-enableddevices.

The core network 106 may facilitate communications with other networks.For example, the core network 106 may provide the WTRUs 102 a, 102 b,102 c with access to circuit-switched networks, such as the PSTN 108, tofacilitate communications between the WTRUs 102 a, 102 b, 102 c andtraditional land-line communications devices. For example, the corenetwork 106 may include, or may communicate with, an IP gateway (e.g.,an IP multimedia subsystem (IMS) server) that serves as an interfacebetween the core network 106 and the PSTN 108. In addition, the corenetwork 106 may provide the WTRUs 102 a, 102 b, 102 c with access to thenetworks 112, which may include other wired or wireless networks thatare owned and/or operated by other service providers.

FIG. 1E is a system diagram of the RAN 104 and the core network 106according to an embodiment. The RAN 104 may be an access service network(ASN) that employs IEEE 802.16 radio technology to communicate with theWTRUs 102 a, 102 b, 102 c over the air interface 116. As will be furtherdiscussed below, the communication links between the differentfunctional entities of the WTRUs 102 a, 102 b, 102 c, the RAN 104, andthe core network 106 may be defined as reference points.

As shown in FIG. 1E, the RAN 104 may include base stations 140 a, 140 b,140 c, and an ASN gateway 142, though it will be appreciated that theRAN 104 may include any number of base stations and ASN gateways whileremaining consistent with an embodiment. The base stations 140 a, 140 b,140 c may each be associated with a particular cell (not shown) in theRAN 104 and may each include one or more transceivers for communicatingwith the WTRUs 102 a, 102 b, 102 c over the air interface 116. In oneembodiment, the base stations 140 a, 140 b, 140 c may implement MIMOtechnology. Thus, the base station 140 a, for example, may use multipleantennas to transmit wireless signals to, and receive wireless signalsfrom, the WTRU 102 a. The base stations 140 a, 140 b, 140 c may alsoprovide mobility management functions, such as handoff triggering,tunnel establishment, radio resource management, traffic classification,quality of service (QoS) policy enforcement, and the like. The ASNgateway 142 may serve as a traffic aggregation point and may beresponsible for paging, caching of subscriber profiles, routing to thecore network 106, and the like.

The air interface 116 between the WTRUs 102 a, 102 b, 102 c and the RAN104 may be defined as an R1 reference point that implements the IEEE802.16 specification. In addition, each of the WTRUs 102 a, 102 b, 102 cmay establish a logical interface (not shown) with the core network 106.The logical interface between the WTRUs 102 a, 102 b, 102 c and the corenetwork 106 may be defined as an R2 reference point, which may be usedfor authentication, authorization, IP host configuration management,and/or mobility management.

The communication link between each of the base stations 140 a, 140 b,140 c may be defined as an R8 reference point that includes protocolsfor facilitating WTRU handovers and the transfer of data between basestations. The communication link between the base stations 140 a, 140 b,140 c and the ASN gateway 215 may be defined as an R6 reference point.The R6 reference point may include protocols for facilitating mobilitymanagement based on mobility events associated with each of the WTRUs102 a, 102 b, 100 c.

As shown in FIG. 1E, the RAN 104 may be connected to the core network106. The communication link between the RAN 104 and the core network 106may defined as an R3 reference point that includes protocols forfacilitating data transfer and mobility management capabilities, forexample. The core network 106 may include a mobile IP home agent(MIP-HA) 144, an authentication, authorization, accounting (AAA) server146, and a gateway 148. While each of the foregoing elements aredepicted as part of the core network 106, it will be appreciated thatany one of these elements may be owned and/or operated by an entityother than the core network operator.

The MIP-HA may be responsible for IP address management, and may enablethe WTRUs 102 a, 102 b, 102 c to roam between different ASNs and/ordifferent core networks. The MIP-HA 144 may provide the WTRUs 102 a, 102b, 102 c with access to packet-switched networks, such as the Internet110, to facilitate communications between the WTRUs 102 a, 102 b, 102 cand IP-enabled devices. The AAA server 146 may be responsible for userauthentication and for supporting user services. The gateway 148 mayfacilitate interworking with other networks. For example, the gateway148 may provide the WTRUs 102 a, 102 b, 102 c with access tocircuit-switched networks, such as the PSTN 108, to facilitatecommunications between the WTRUs 102 a, 102 b, 102 c and traditionalland-line communications devices. In addition, the gateway 148 mayprovide the WTRUs 102 a, 102 b, 102 c with access to the networks 112,which may include other wired or wireless networks that are owned and/oroperated by other service providers.

Although not shown in FIG. 1E, it will be appreciated that the RAN 104may be connected to other ASNs and the core network 106 may be connectedto other core networks. The communication link between the RAN 104 theother ASNs may be defined as an R4 reference point, which may includeprotocols for coordinating the mobility of the WTRUs 102 a, 102 b, 102 cbetween the RAN 104 and the other ASNs. The communication link betweenthe core network 106 and the other core networks may be defined as an R5reference, which may include protocols for facilitating interworkingbetween home core networks and visited core networks.

In the context of D2D communications, the link adaptation mechanism maybe used to optimize a D2D link. The D2D link may be a communication linkbetween each D2D WTRU. One or more implementations for link adaptationand for measurements associated to link adaptations may be provided. Thelink adaptation problem in the context of D2D communications may bedivided into a number of parts. One problem, for example, may involvehow the D2D link channel measurements may be performed by the D2D WTRUs,e.g., what channels may be measured and the associated procedures.Another problem may involve how the D2D WTRUs determine the actualtransmission parameters from the measurements. The D2D WTRUs may carryout scheduling, in which case one or more of the transmission parameterssuch as RB allocation, modulation and coding scheme (MCS), and/or rankmay be calculated. Another problem may involve determining the signalingmeans and procedures to exchange the information.

Methods, systems, and instrumentalities are described herein todetermine link adaptation parameters. The link adaptation parameters maybe determined by a transmitting WTRU and/or a receiving WTRU. Themethods, systems and instrumentalities described herein are exemplaryand may be applicable to contexts other than the contexts in which theyare disclosed.

FIG. 2 illustrates a diagram showing an example of devices (e.g., atransmitting device 202, a receiving device 204, a measuring device 206,and/or a controlling device 208). The transmitting device 202 may be adevice that may transmit data. The transmitted data may be subject tolink adaptation. A receiving device 204 may be a device that may receivedata. The received data may be subject to link adaptation. A measuringdevice 206 may perform channel measurements for link adaptationpurposes. A controlling entity 208 may refer to an entity in a device ornetwork node that may determine at least one transmission parameter forlink adaptation purposes. The controlling entity 208 may be a part ofthe controlling device or an eNB. The terms controlling entity andcontrolling device may be used interchangeably. A forward channel or adirect channel may be a target of link adaptation, e.g., from atransmitting device to a receiving device. A reverse channel may be in areverse direction from the direct or forward channel, e.g., from areceiving device to a transmitting device. A downlink may be a link(e.g., a traditional link) from a network node, e.g., an eNB, and adevice. Link information may include a set of transmission parameters,e.g., a grant, or information that may help the transmitter to decideits transmission parameters, e.g., CSI. A traditional link (TRL) may bea legacy channel transmission between a network node, e.g., an eNB, anda device (e.g., a WTRU). A D2D link or a cross link (XL) may be a linkor a channel connection between at least two WTRUs that may directlycommunicate, transmit, and/or receive data. A D2D WTRU may be a device(e.g., a WTRU) that may be configured to be a part of a D2D link.

FIG. 3 illustrates an example case of FIG. 2, where the receiving device204 may be the measuring device 206. The measuring device may performchannel measurements. The measuring device may estimate a channel fromat least one channel measurement resource. The channel measurementresource may include a known signal (e.g., by a transmitting D2D WTRUand receiving D2D WTRU) transmitted during known REs and subframes,possibly with zero power. The measuring device may estimate the channeland/or the interference from the same channel measurement resource ormay use a separate channel measurement resource for this purpose.

The measuring device (e.g., for desired channel measurement and/orinterference measurement) may be the receiving device. In this case, thetransmitting device may transmit reference signals or mutes for channelestimation. The measurements may provide an estimate of the forwardchannel.

The measuring device may be the transmitting device. In this case, thereceiving device may transmit reference signals or mutes for channelestimation. The measurements may provide an estimate of the reversechannel, and the measuring device or controlling device may derive anestimate of the forward channel based on channel reciprocity.

As illustrated in FIG. 3, the transmitting device may transmit data anda reference signal on the channel measurement resource. It should beunderstood that the examples disclosed herein may also apply to the casewhere the measuring device may be a transmitting device. For a givendevice, its role (e.g., receiving vs. transmitting) may change on atransmission time interval (TTI) basis. For example, a given device maybe configured to be transmitting on a subset of TTIs and receiving onanother subset of TTIs. A WTRU may change its role dynamically.

To simplify the disclosure, some examples are disclosed in the contextof a single reference signal. It should be understood that the disclosedexamples may also apply to the case where the WTRUs may be configuredwith more than one reference signal process with an associated set ofparameters. In an example, a reference signal process may include a setof parameters characterizing a set of reference signals, for example,reference signal resource/patterns, periodic cycles, transmission power,type of measurement to perform, and/or report destination (e.g., othertransmitting device/controlling entity or eNB).

A measuring device may measure a reference signal from which it mayestimate the desired channel. If the receiving device is the measuringdevice, the transmitting device may be configured to transmit thereference signal over a known set of channel measurement resources. Ifthe transmitting device is the measuring device, the receiving devicemay be configured to transmit the reference signal on the channelmeasurement resource.

The measuring device (e.g., assuming that the receiving device is themeasuring device) and the transmitting device may be configured with aset of channel measurements resources, e.g., via RRC configuration. Theset of channel measurement resources may include a set of subframes,slots, and resource elements (REs), etc. The transmitting device maytransmit the reference signal on the channel measurement resources. Thereference signal may be a signal or a combination of a number ofsignals. For example, the reference signal may include a referencesignal defined for an LTE uplink (UL), such as SRS or UL DM-RS, based ona Zadoff-Chu (ZC) sequence. An SRS-like signal may be defined for thepurpose of D2D CSI measurement. This SRS-like signal may, for instance,occupy a different symbol or more than one symbol in a subframe. Asanother example, an UL DM-RS-like signal may be defined, for example,using a different or more than one symbol in a subframe. The referencesignal may include a reference signal defined for an LTE downlink (DL),such as CSI-RS or DL DM-RS. The reference signal may include a discoverysignal, e.g., used for device to device discovery. A discovery signalmay be a signal a D2D WTRU may use to advertise its capabilities and/orsearch for other devices capable of D2D communication. The referencesignal may include a preamble or postamble signal, e.g., a specialsequence transmitted before and/or after data transmission. Thereference signal may include other types of reference signals.

The measuring device may be configured to estimate the channel based onthe received reference signal, e.g., as transmitted on the known set ofchannel measurement resources. The measuring device may be atransmitting device or a receiving device.

FIG. 4 is a diagram illustrating channel measurement and reporting ofCSI for the subcarriers overlapping with the allocated D2D communicationset of subbands (herein referred to as D2D bandwidth). As illustrated inthe FIG. 4, a WTRU may be configured to receive the signal from one ormore antennas 420 and process the RF signal so that it can be digitizedto baseband using the RF/ADC functionality 402. At 404, the WTRU mayremove the Cyclic Prefix. Using a serial to parallel converter 406, theWTRU may convert the samples into a parallel form and may apply anM-point DFT 408 to recover the sub-carriers associated with the widebandreference signal 410. Using a reference signal sub-carrier selectionmechanism 412, the WTRU may select a subset of the sub-carriersassociated to D2D communications 414. Using the channel measurementprocessing 416, the WTRU may determine and report the channel stateinformation (CSI) associated with the D2D communications sub-carriers418. The term bandwidth may be interchangeably used herein with set ofsubbands. The reference signal may be configured to occupy a largerbandwidth (wideband reference signal) than that occupied by the D2Dcommunications. In that case, the measuring device may be configured toreport the CSI for the part overlapping the allocated D2D communicationbandwidth.

The measuring device may be configured to measure on a set of channelmeasurement resources. For example, the measuring device may measureinterference on a channel measurement resource. These channelmeasurement resources may be the same or different from the channelmeasurement resources used for desired channel estimation. In anexample, the measuring device may use one or more of the referencesignals for estimating the interference of the transmitted signal andbase its estimate on, for example, the variance of the signal. Themeasuring device may be configured to measure the interference on a setof resource elements in a set of subframes in which the desired devicemaybe known not to transmit, e.g., zero power. For example, themeasuring device may be configured to measure the interference on awhole subframe, e.g., during which the transmitting device is known notto transmit. The measuring device may be configured to measure theinterference on a zero-power SRS, e.g., where the transmitting devicedoes not transmit in the last symbol of a certain subframe or set ofsubframes. The measuring device may be configured to measure theinterference on a zero-power symbol, e.g., where the transmitting devicedoes not transmit in a specific time symbol of a certain subframe or setof subframes. The measuring device may be configured to measure theinterference on a zero-power subcarrier or resource block(s) or on azero-power CSI-RS. The measuring device may be configured to measure theinterference on a set of resource elements in a set of subframes inwhich at least one interferer is known to transmit a known signal.

The measuring device may be configured to measure one or more referencesignals. The reference signal may span the bandwidth occupied by the D2Dcommunications. The measuring device may be configured to identify thesereference signals, for example, based on a scrambling sequenceinitialized with a known value provided during configuration, the use ofa set of resource elements or symbols or subcarriers, and/or otherfactors.

The reference signals may be transmitted and/or received at specifictime instants, e.g., to measure interference. The measuring device andthe transmitting device may be configured, for example, via RRCsignaling with one or more sets of subframes or sequences of slots orsubframes over which the reference signals are transmitted (for thetransmitting device) and/or may be measured (for the measuring device).These reference signals may include a zero power transmission, forexample, to allow the measuring device to measure the interference.

In an example, a WTRU (e.g., a transmitting device or a receivingdevice), may be configured to maintain a periodic measurementtransmission or reception activation and deactivation status. The WTRUmay be activated or deactivated via a L1 (e.g., PDCCH) or L2 (e.g.,MAC-CE) message, for example, originating from the controlling device.The controlling device may be an WTRU or an eNB. The transmitting devicemay be configured to transmit the reference signals according to theconfigured periodic schedule when it is activated. The transmittingdevice may be configured to not transmit the periodic reference signalwhen it is deactivated, or transmit the periodic reference signalaccording to a longer period. The longer period may be configured ordetermined by the WTRU based on fixed rules. The receiving device may beconfigured to receive and act on the reference signals according to aconfigured periodic schedule when it is activated. The receiving devicemay be configured to not monitor the periodic reference signal, and notact on it, or may be configured to monitor the periodic signal accordingto the longer period when it is deactivated.

A WTRU may be configured to determine the activation/deactivation statusautonomously. For example, a transmitting WTRU may be configured toactivate the reference signal periodic transmission when one or more ofthe following conditions or triggers are met, either individually or inany combination. The transmitting WTRU may be configured to activate thereference signal periodic transmission when the reference signalperiodic transmission is deactivated or the transmitting WTRU had anempty D2D buffer (e.g., a buffer containing data destined to D2Dcommunications, e.g., data from D2D logical channels) and now the D2Dbuffer is no longer empty, or the transmitting WTRU D2D buffer is abovea configured threshold. The transmitting WTRU may be configured toactivate the reference signal periodic transmission when a D2Dtransmission session is configured by higher layers. For example, thetransmitting device may be configured (e.g., via RRC configuration) bythe controlling device or by the eNB to start a D2D transmissionsession. The application layer may indicate initiation of a D2Dsession/transmission. The transmitting WTRU may be configured toactivate the reference signal periodic transmission when the frequencyresource allocation has changed, e.g., the WTRU has received anindication from the controlling entity or from the eNB that the D2Dcommunication is taking place on a different or new set of PRBs.

The transmitting WTRU may be configured to activate the reference signalperiodic transmission when D2D session has been activated ordeactivated. For example, the transmitting WTRU may be configured toactivate the reference signal periodic transmission when the WTRU haschanged its role, for example, from a receiving device to a transmittingdevice or vice versa.

The transmitting WTRU may be configured to activate the reference signalperiodic transmission when the rate of HARQ retransmissions on the D2Dlink is above a threshold. The transmitting WTRU may be configured toactivate the reference signal periodic transmission when the D2D datalink block error rate (BLER) is above a threshold. For example, thetransmitting WTRU may be configured to activate the reference signalperiodic transmission when a channel state information invalidity timerhas expired. In an example channel state information validity timer, thetransmitting WTRU may be configured with a channel state informationvalidity timer value. The transmitting WTRU may be configured to startthe timer when the WTRU deactivates the reference signal periodictransmission. The transmitting WTRU may be configured to stop and resetthe timer when the reference signal periodic transmission is activated.

A transmitting WTRU may be configured to activate the reference signalperiodic transmission in response to reception of a signal from ameasuring device. The signal may include the PUCCH and/or the PUSCH, forexample, transmitted from the measuring device. The transmitting WTRUmay be configured to activate the reference signal periodic transmissionin response to an ACK or NACK signal, e.g., when the ACK or NACK signalis not associated to a particular active data transmission. Theconditions or triggers described herein may be used for other purposesin any order or combination, including, e.g., a trigger to transmit anaperiodic reference signal.

The transmitting WTRU may be configured to deactivate the referencesignal periodic transmission when one or more of the followingconditions or triggers are met in any order or combination. Thetransmitting WTRU may be configured to deactivate the reference signalperiodic transmission when the reference signal periodic transmission isactivated and the transmitting WTRU D2D buffer is empty or below aconfigured threshold and/or an inactivity timer has expired. In anexample inactivity timer, the transmitting WTRU may be configured withan inactivity timer value. The transmitting WTRU may be configured tostart the timer when the D2D buffer is empty and may be configured tostop and reset the timer when the D2D buffer is no longer empty. Thetransmitting WTRU may be configured to deactivate the reference signalperiodic transmission when the D2D transmission session is terminated orhanded over by higher layers, e.g., when the transmitting device isconfigured (e.g., via RRC configuration) by the controlling device or bythe eNB to stop D2D transmission or when the application layer indicatestermination of the D2D session/transmission.

When the reference signal periodic transmission is activated, thetransmitting WTRU may be configured to perform one or more of a numberof actions in any order or combination. The transmitting WTRU may beconfigured to transmit an indication of the reference signal periodictransmission activation. The WTRU may transmit a control channelindicating activation of the reference signal periodic transmission. Thereference signal may be received by the eNB or a controlling entity. Thereference signal may be carried, for example, using a PUCCH (e.g., aPUCCH carrying D2D-related information), or as part of a PUSCH (e.g.,via MAC-CE or other L2 signaling means). The WTRU may transmit thereference signal over a non-D2D resource at the signal power associatedwith the conventional radio link, e.g., to the eNB. The eNB mayconfigure the receiving WTRU. The signal may be sent to the receivingWTRU and may be carried on a PUCCH-like channel, e.g., a channel for D2Dcontrol information, or on a PUSCH-like channel, e.g., via L2 signaling.The transmitting WTRU may transmit the reference signal over D2Dresources at the signal power configured for D2D communications.

The transmitting WTRU may start transmission of the reference signal atthe next occasion according to the configured schedule. The transmittingWTRU may be configured to start transmission of the reference signal atthe first occasion according to the reference signal transmissionschedule, for example after expiration of a wait or validity timer. Thetransmitting WTRU may be configured to start data transmission, forexample, waiting for the CSI feedback from the receiving WTRU beforeinitiating data transmission. The transmitting WTRU may transmit datausing a predefined transport block size (TBS) and associated MCSparameters. The transmitting WTRU may transmit data until it receivesCSI feedback. The transmitting WTRU may be configured to stop and reseta channel state information invalidity timer.

When the reference signal periodic transmission is deactivated, thetransmitting WTRU may be configured to perform one or more of a numberof actions in any order or combination, e.g., start a channel stateinformation invalidity timer.

A receiving WTRU may determine activation or deactivation status of theperiodic reference signals. For example, the receiving WTRU may beconfigured to receive an activation status message from the transmittingWTRU. The receiving WTRU may be configured to request activation ordeactivation of the periodic reference signals. The receiving WTRU maybe configured to transmit the request for activation or deactivation tothe controlling device or eNB, or to the peer transmitting WTRU. Therequest may be carried on a PUCCH or PUSCH (e.g., a new PUCCH or PUSCH),e.g., via MAC-CE. The receiving WTRU may further start monitoring thereference signal a fixed amount of time after transmission of therequest or a fixed amount of time after it has received confirmationthat the request has been received correctly.

In another example, the receiving WTRU may be configured with aninactivity timer. The receiving WTRU may start the inactivity timer, forexample, when no data is received on the D2D data resource. Thereceiving WTRU may be configured to stop and reset the inactivity timerwhen data is received on the D2D data resource. The receiving WTRU maybe configured to deactivate reception of the periodic reference signal,e.g., upon timer expiration.

When the periodic reference signal state has been deactivated, themeasuring device may be configured to perform one or more of a number ofactions in any order or combination. The measuring device may beconfigured to stop monitoring the reference signal and/or stoptransmitting the CSI feedback. The measuring device may be configured tomonitor the reference signal according to a longer period and report CSIfeedback at the appropriate longer period. The measuring device may beconfigured to transmit an indication of the reference signaldeactivation. For example, the measuring device may transmit theindication to the transmitting device using a special PUCCH format or aspecial MAC-CE on the PUSCH to carry the indication, using, forinstance, the power and resources associated with the D2D transmission.The measuring device may transmit the indication to the controllingdevice or eNB. The measuring device may be configured to transmit, forexample, using a special PUCCH format or a special MAC-CE on the PUSCHto carry the indication, using, for instance, the power and resourcesassociated with the link to the eNB or a controlling device. Themeasuring device may further be configured to stop and reset aninactivity timer upon deactivation. The measuring device may beconfigured to start the inactivity timer when the periodic referencesignal is activated.

The measuring device may be configured to measure the channel and/orinterference based on aperiodic triggers. The transmitting device may beconfigured to transmit reference signals based on aperiodic triggers.For example, the transmitting device and the measuring device may beconfigured with a set of reference signal parameters, including, forexample, the reference signal length and/or bandwidth, root sequenceand/or cyclic shift (in the case of Zadoff-Chu sequences). For example,the reference signal may be configured to span a larger bandwidth thanwhat the controlling device or the eNB may have configured for D2Dcommunications (e.g., wideband reference signal). This may allow thecontrolling device or eNB to determine how to schedule D2Dcommunications over different sets of subbands for example to improvesystem throughput. The devices may be configured with one or morereference signal parameters in a message from the eNB. The devices maybe pre-configured with a subset of the aperiodic reference signalparameters (e.g., sequence root, cyclic shift for the case of Zadoff-Chusequences, transmission power, etc.), e.g., via RRC signaling. Thedevices may be indicated the parameters for transmission and/orreception of the reference signal (e.g., index to set of PRBs, or otherparameters), e.g., as part of L1/L2 signaling. In an example, thedevices may be indicated an index corresponding to a predefined set ofparameters (e.g., set of subcarriers, PRBs, etc.) completing theparameters to use for transmitting/receiving the reference signal.

The measuring device may request an aperiodic reference signal. In anexample, the measuring device may be configured to request thetransmitting device to transmit a reference signal. For example, uponreceiving a request signal, the transmitting device may be configured totransmit a reference signal or, alternatively, zero power, if the signalrequests interference measurement. The reference signal or zero powermay be transmitted at an occasion (e.g., the first occasion). Thetransmitting device may be configured to transmit the reference signal aconfigured amount of time after receiving the request. This approach mayfacilitate synchronizing both devices in time.

One or more of a number of approaches to transmitting the referencesignal may be taken. For example, the transmit device may be configuredto transmit the reference signal N subframes after receiving the requestmessage. The value of N may be configured by RRC signaling or fixed inapplicable specifications. A transmitting device may be configured totransmit the reference signal at least N subframes after receiving therequest message during the next allowed slot or subframe. The value of Nmay be configured by RRC signaling or may be fixed in applicablespecifications. The allowed slot or subframe may be configured by thenetwork, e.g., a subset of the slots or subframes may be used for theaperiodic reference signal transmission. In another example, the allowedslots or subframes may be the same as the ones configured for periodicreference signal transmission.

The measuring device may transmit a reference signal request. Forexample, a signal carrying the request for the reference signaltransmission may be transmitted by the measuring device in a number ofdifferent ways. The measuring device may be configured to transmit anexplicit request signal using, for example, one or more of a number ofapproaches. For example, the measuring device may be configured to use aspecial resource, e.g., on the PUCCH or PUSCH, to transmit a referencesignal request (RSR). A similar approach using the SR with PUCCH format1 may be used. The measuring device may be configured to transmit theRSR using a field (e.g., a new field) or replacing an existing bit orfield on, for example PUCCH format 3. As another example, a field in acontrol channel, (e.g., a new control channel) transmitted on a PDCCH orePDCCH may be used to carry the RSR, for example.

The measuring device may be configured to transmit an RSR, for example,when the D2D link is activated or is being activated, and/or when themeasuring device buffer is above or below a configured threshold.

It should be appreciated that, while examples described herein may usethe receiving device as the measuring device, the examples may beapplicable to other contexts. The reference signal request may bereceived from the receiving device, e.g., where the reference signal maybe transmitted from the transmitting device; the transmitting device,e.g. where the reference signal may be transmitted from the receivingdevice, thereby exploiting reciprocity in the link; or the controllingdevice, e.g., an eNB from the downlink.

The transmitting device may be configured to initiate transmission ofthe reference signal, for example, in order to obtain CSI for the D2Ddata transmission from the receiving device. FIG. 5 illustrates anexample 500 for transmitting an aperiodic reference signal. Transmissionof an aperiodic reference may involve one or more of the actionsdepicted in FIG. 5. The actions may be in any order or combination. At502, the aperiodic reference signal transmission may be initialized ortriggered. The initiation of the transmission of the aperiodic referencesignal may be triggered by the controlling device or by the eNB. Thismay be used for allowing the controlling device to obtain a measurementof the D2D channel for scheduling radio resources on a system level. Theinitiation of the transmission of the aperiodic reference signal may betriggered by the transmitting device, e.g., when one or more of theconditions for activating the reference signal periodic transmission ismet.

At 504, an indication of the aperiodic reference signal transmission maybe transmitted or received. For example, the transmitting device may beconfigured to transmit an indication of the aperiodic reference signaltransmission to the measuring device. The examples of signalingapproaches disclosed herein in connection with periodic transmission ofreference signals may be applicable. For example, the transmittingdevice may receive an indication by the controlling device or the eNB totransmit an aperiodic reference signal. The measuring device may alsoreceive an indication from the controlling device or eNB that thereference signal may be transmitted by the transmitting device. Thisindication may be carried, for example, over the PDCCH. In an example,this signal may be used to reach both transmitting and receivingdevices, for example, using a special RNTI common to both transmittingand receiving devices. The transmitting device may be configured totransmit the aperiodic reference signal immediately or at a configuredamount of time after the trigger, or when it transmits or receives theindication. Similarly, the measuring device may be configured in thesame way. The transmitting device may be configured to wait for aresponse from the measuring device, e.g., before starting thetransmission of the reference signal. At 506, the aperiodic referencesignal may be transmitted or received.

At 508, an associated measurement may be transmitted or received. Forexample, the measuring device may be configured to perform a measurementon the received reference signal and send feedback (e.g., the CSI) tothe transmitting device. For example, the feedback may be transmitted ona PUCCH transmitted over D2D resources at the power configured for D2Dcommunications. The measuring device may be configured to perform ameasurement on the received reference signal and send feedback (e.g.,the CSI) to a controlling device or an eNB. This may be achieved, forexample, using an PUCCH, e.g., using an existing or a new PUCCH formattransmitted over resources associated with the controlling device oreNB, at an appropriate power level for proper reception.

Some examples may involve multiple reference signal processes. In anexample, the devices may be configured with more than one referencesignal processes (e.g., periodic and/or aperiodic). For example, atransmitting device may be configured to transmit, in a subframe, aperiodic reference signal and at the same time may be configured totransmit an aperiodic reference signal (e.g., probing a different partof the spectrum, for example from a different reference signal process).This may be, for example, to facilitate the eNB to be able to change theD2D link resources to a different set of PRBs.

In the case of overlapping reference signal transmission/reception orconflict between reference signal occasions and other signals, thedevices may be configured with a set of rules, to determine theprecedence. A number of example rules may be applied in any order orcombination. For example, priority may be given to an aperiodicreference signal over a periodic signal. As another example, prioritymay be given to reference signals associated with measurements to bereported to the eNB. Priority may be given to PUCCH (e.g., carrying CSIinformation) over periodic reference signals. As another example,priority may be given to an aperiodic reference signal over PUCCH/PUSCHtransmission when they conflict.

The reference signal may be designed to coexist with other signals. Insuch cases, the priority rules may be inapplicable, as there may be noconflict. In another example, the receiving device may be configured toreport one or more types of measurements from the same reference signal.The transmitting device may be configured with a single reference signalprocess, which may, for instance, cover a larger bandwidth than the setof subbands allocated for D2D communications. For example, thetransmitting device may be configured for conventional SRS transmission(e.g., potentially hopping in frequency). The measuring device may beconfigured with more than one reference signal process for reportingpurposes, based on the same transmitted reference signal. The measuringdevice may be configured to report the CSI from the set of subbands inthe allocated D2D bandwidth (e.g., in the set of subbands allocated forD2D communications) separately from the other set of subbands. In anexample using the SRS, the measuring device may determine theappropriate set of SRS sequence parameters (ZC, cyclic shift, hoppingpattern) for the set of subbands allocated to D2D communications fromthe configured (larger bandwidth) SRS set of parameters. The measuringdevice may measure and report the CSI for the D2D related subbands whenthe reference signal from the SRS is known to be transmitted in the setof D2D subbands.

Determining the link adaptation parameters for the D2D link may have anumber of challenges, including, for example, varying levels ofinterference due to a dynamic D2D link schedule and traffic load. Theremay be mutual interference when sharing resources with TRL. Anotherchallenge may be less accurate channel quality indication (CQI)measurement compared with TRL UL, and/or inaccurate SINR estimation andCQI prediction due to channel estimation error due to duplexing,quantization error, inherent delay of transmission and processing,and/or receiver capability.

To handle these challenges, a number of approaches may be taken. Forexample, the approaches may include, measurements and predictionconsidering channel reciprocity for D2D link forward and reversechannels, iteratively adaptive adjustments in CQI prediction,differential CQI feedback, prediction, and/or tracking, CSI PMI feedbackwith an additional delta between PMI and the ideal CSI. The approachesmay, e.g., further include a comparison between a D2D link and TRL UL todetermine whether the D2D link offers a higher throughput, interpolationand prediction considering quantization error and other impairments,hierarchical feedback (e.g., more feedback information on a strongertransmitter), or CSI-RS and DM-RS supported CSI feedback.

Some approaches may be based on loop control and adaptation. Linkadaptation for the D2D link may combine fast inner loop link adaptation(ILLA) and slow outer loop link adaptation (OLLA). The ILLA maycalculate the suitable MCS for the WTRU based on the mapping between thereceived CQI (e.g., measured SINR from the reference symbols) to themost appropriate MCS for an allocation. The OLLA may adapt the MCSselection to provide a BLER. The target BLER may be set to provide anacceptable or optimal performance, e.g., depending on whetherretransmission mechanisms like (H)ARQ are utilized.

The measuring device (e.g., the receiving device or the transmittingdevice) may perform calculation, for example, based on measurements ofthe reference signals. Calculations may rely on the eNB taking intoaccount other D2D links in the proximity of the cell.

Exponential effective signal to noise ratio (SNR) mapping (EESM) may beused to translate a block of varying channel SINR values from subbandsto an effective wideband SINR value, of which the BLER may be equivalentto that corresponding to the block of varying SINR. The OLLA may imposean offset, referred to as the link adaptation margin, which may besubtracted from the SINR estimate from the CQI measurements before beingused by the inner loop LA. The OLLA may control the experienced averageBLER for the first transmissions. For example, the offset margin may beincreased, if the first transmission is an ACK and decreased if it is anNACK. Positive offsets, e.g., for situations where the OLLA starts withmore conservative MCSs, may be changed at a relatively lower pace thanin negative offsets, where the MCSs may be more aggressive.

The link adaptation margin may be a fixed value, and adaptively adjustedwith an adaptive bias for each WTRU through fast and/or slow adjustment.For TRL UL (e.g., more stable situations), a fixed link adaptationmargin may perform better. In this case, functions may be available todisable differential link adaptation such that a fixed optimized linkadaptation margin is used. For example, the TRL UL performance may beworse with differentiated link adaptation than with a fixed optimizedlink adaptation margin. This could be because the TRL UL SINR estimationmay be more accurate than in the D2D link. In low estimation errorvariance, frequent SINR adjustments may make the situation worse.

The link adaptation may be performed on a slow basis, for example, withthe same rate of the power control commands to exploit the slowlychanging channel variations, or on a faster basis, for example for eachof TTIs, to exploit the high instantaneous SINR conditions. Fast linkadaptation may be used to adjust the link adaptation margin of each userbased on the ACK/NACK feedback for the last transmission. Slow linkadaptation may be applied by windowing the ACK/NACK during a timeperiod. The calculated packet error BLER may be used, e.g., based on thetraffic model and scheduler.

Some approaches may be based on an announce scheduler. For example, tosolve the bursty interference due to dynamic D2D link scheduling, anannounce scheduler may pre-allocate the D2D link pairs before the SINRand CQI measurement. The pre-allocated D2D links may provide the WTRUinformation to predict the future interfered D2D links, e.g., D2D linksthat may have interference in the future. With the extracted schedulinginformation, the WTRU may predict CQI when future D2D link interferencemay appear.

The announce scheduler may be changed to assign different D2D links witha predetermined pattern so the pattern information may not be fedforward in some scenarios. The announce scheduler may apply to a relayWTRU so that the WTRU may use the information to determine whether theD2D link is better than TRL UL and/or the relay link.

To facilitate the announce scheduler, some RSs may be used that maycontain information of interfered D2D link power and spatial property.This information may be used by the WTRU with a MMSE type receiver tocalculate CQI. Some controlling entities may be used to conveyinformation, such as modulation type, which may be used by the WTRU withan ML receiver. The control information may be accessible to multipleWTRUs, e.g., each of the WTRUs, e.g., using a broadcast approach. Thescheduling decisions may be exchanged between a certain set of D2D linksvia TRL through controlling entities or combined with channel estimate(ChEst) for multiple D2D links. The WTRU may use techniques that mayallow a transmitter to estimate path loss to a receiver, e.g., bypassively overhearing link messages sent by the receiver.

The WTRU may avoid or mitigate interference through D2D linkcoordination. For example, this may involve precoding or beamformingwith appropriate receiver filters in the case of multiple antennas andfeedback of the best and worst D2D link pair choices. CSI independentprecoding may be used to pre-assign D2D links assisted with low overheadCSI dependent precoding and power control. The D2D link coordination mayinvolve inter-D2D link interference rejection techniques. A receiver(e.g., an advanced receiver) may be used to reject interference and toamplify a desired signal by weighted signals in combining to increase ormaximize SINR. Precoding or beamforming and CIS information frommultiple D2D links may be accessible through controlling entities. Thiscoordination may occur in a centralized or a decentralized manner.

Calculation of CQI and MCS may be ignored to save power, e.g., when highinterference occurs for some subbands and/or some TTI. For example, inthe presence of high D2D link interference, some bandwidth may berestricted, e.g., colliding PRBs may be banned from use in the neighborD2D link or may be used with restrictions (e.g., with lower power).Scheduling may be delayed, e.g., some WTRUs in the interfering orinterfered D2D links may be postponed.

To save reverse channel overhead, wideband CQI reporting may be used asa main feedback mode. In this case, SINR from each of the carriers maybe averaged and reported as one CQI representing the entire measuredbandwidth. Subband CQI reporting may involve averaging some carriersdetermined by the parameter frequency granularity conveyed by thecontrolling entities or known in advance by the WTRUs. Due to thedynamic D2D link properties, the D2D link interference may be weaklycorrelated with that in subsequent subframes, and the usefulness of CQIfeedback for low SINR D2D links may be reduced. A less frequentscheduling decision may be used. A filtering may be used in CQI feedbackor estimation to average out temporal interference variations.

In an example, similar to MCS determination, which may adapt to thepredicted CQI (from estimated SINR) by selecting the most appropriateMCS, the resource block may adaptively allocate the bandwidth, which maybe scalable to the service types, cell load, and/or power limitationsfor different D2D links. Adaptive bandwidth allocation may select theappropriate portions, such as in a subband, of the bandwidths todifferent D2D links which may have different cell loads. The adaptivebandwidth allocation may be similar to multi-link diversity.

In an example, a controlling entity for link adaptation (LA) maydetermine one or more transmission parameters for the transmittingdevice. Depending on the system architecture, the controlling entity maybe located in the eNB, a controlling device, the measuring device, thetransmitting device, and/or the receiving device. For conciseness, thefollowing solutions may be disclosed under the assumption of anarchitecture (e.g., where the controlling entity may be located in thereceiving device). It should be understood, however, that the disclosedexamples may be applicable to architectures other than the architecturein the context of which they are disclosed.

The controlling entity in the receiving device (or the receiving devicedirectly), for example, may send (e.g., directly) link adaptationinformation parameters to the transmitting device. The receiving devicemay use one or more of PUCCH (e.g., using PUCCH format 3), PUSCH (e.g.,using a known control information area), PDCCH, and/or ePDCCH totransmit the information. The receiving device may use a channelspecifically designed for transmitting link adaptation informationparameters.

The controlling entity in the receiving device may transmit the linkadaptation information to the transmitting device. In another example,the controlling entity in the receiving device may transmit the linkadaptation information to the network, e.g., the eNB. The network inturn may relay this information to the transmitting device.

The controlling entity may be located in the network, for example, inthe eNB controlling the D2D link, on in a separate controlling device.The receiving device or the measuring device may transmit measurementinformation to the controlling entity in the network (e.g., to the eNB).The eNB may calculate the link adaptation parameters and signal theinformation to the transmitting device. The receiving device may receiveand decode the link adaptation parameters transmitted from the eNB inorder to decode the associated data transmission.

The transmitting device may receive the transmission parameters from thecontrolling entity, which may be either in the receiving device or inthe eNB. The link information provided by the controlling device mayinclude MCS, RBs, a grant (e.g., in terms of a number of bits), acarrier index, etc. For example, the link information carries theinformation transmitted on DCI format 0 or 4.

In an example, the transmitting entity may get a subset of the linkadaptation parameters and/or an indication from the controlling entityon the network or receiving entity. The transmitting entity maydetermine on its own the remaining parameters to obtain the full set oftransmission parameters. For example, the transmitting entity mayreceive the link information, which may consist of the CSI (e.g., thenumber of layers, the precoding, the CQI for each codeword, and/orfrequency-selective parameters). Based on the link information, thecontrolling entity on the transmitting entity may calculate thetransport block size and MCS. The calculation may be further based on alogical channel of the data to be transmitted (e.g., using theassociated HARQ profile information) and/or an offset derived from anouter-loop link adaptation. This offset may depend on a BLER experiencedin past transmission, a target for the link (as configured, e.g., by thenetwork via RRC signaling), and/or outer-loop mechanism parameters(e.g., update increment, filter length or window size, etc.).

The entity (e.g., an WTRU or an eNB) transmitting the link information(e.g., partial or complete) may be configured to transmit the linkinformation. The link information may be transmitted on a periodic basisor following one or more triggers. The entity receiving the linkinformation may be configured to receive this information. In an exampleof periodic transmission of link information, the entities may beconfigured with a semi-static period or time offset, for example, viaRRC signaling. The actual period/offset applied may further depend onother parameters.

In an example of transmission of the link information, the D2D WTRU oreNB, depending on the architecture, may be configured to transmit thelink information a fixed amount of time after the reference signal usedfor channel measurement is transmitted. For example, if the receivingentity transmits the link information, the receiving entity may beconfigured to transmit the link information M subframes after thetransmitting entity transmitted the reference signal. The entity may beconfigured to transmit the link information following periodic referencesignal transmission or conversely, following aperiodic reference signaltransmission.

In case of aperiodic transmission of the link information, the linkinformation may be transmitted after reception of a special request fromthe transmitting device. For instance, the receiving device (or thecontrolling eNB) may receive a request from the transmitting device.This request may be transmitted using, for example, L1 or L2 messaging.The receiving device may measure the channel (e.g., the request for linkinformation may be accompanied by an associated reference signaltransmitted by the transmitting entity). The receiving device maycalculate the link information and/or may transmit the link informationto the transmitting entity or to the controlling eNB.

The transmitting device or WTRU may be configured to transmit datafollowing the reception of the link information. The link informationmay be persistent, and the transmitting device may use the linkparameters from the last received link information. The transmittingdevice may use the link information received to issue a singletransmission and the associated HARQ retransmissions. For example, thelink information may last for a single subframe. This may be similar toa transmitting device receiving a new grant for each of thetransmissions (e.g., except the HARQ retransmissions).

The transmitting device may be configured to start or stop transmissioneven if it receives link information, for example, if the transmittingdevice is buffer-limited, or if the transmitting device determines thatvery strong interference is present.

D2D resource allocation or scheduling may be provided. D2Dcommunications may be scheduled or configured on a set of semi-staticsubcarriers. The eNB may not control D2D link adaptation parameters. Itmay be impractical for the eNB to dynamically schedule the D2D resourcesto take advantage of the frequency selective fading. In case ofmulti-user scheduling, the eNB may change the D2D frequencyallocation(s) so that other WTRUs may be scheduled over that set ofresources (e.g., to improve performance). The eNB may determine if theD2D link could use another set of frequency resources. In the case wherethe D2D link may not provide the expected performance, the eNB, forexample, may determine whether or not a different frequency allocationmay resolve the link performance issue. The WTRUs involved in the D2Dcommunications may be configured with a set (e.g., special set) ofreference signal parameters, for example, according to one or morechannel measurement approaches disclosed herein. The measuring devicemay be configured to report to the eNB or controlling device directly,for instance, using the PUCCH or the PUSCH, with the appropriate powerlevel and timing advance (e.g., those associated to the eNB/controllingdevice).

The measuring device may be configured to report periodically orfollowing an aperiodic request. The measuring device may be configuredto measure the reference signal and calculate one or more metricsindicative of the channel quality (e.g., over a set of predefinedsubcarriers or resource blocks (RB)). In an example, the WTRU may reportan index pointing to an entry in a table of metrics. The metric mayinclude an average throughput, e.g., the measuring device may estimatethe average throughput expected from this link assuming a pre-definedtransmit power and/or other pre-defined link parameters. The metric mayinclude a transport block size, e.g., the measuring device may estimatethe transport block size it could receive with a predefined BLER (e.g.,0.1 BLER at first transmission), under specific reception conditions(e.g., similar to CQI). The metric may include a regular CQI, e.g., themeasuring device may be configured to report the CQI as in the legacymode of operations. The metric may include a D2D Reference signalreceive power (D2D-RSRP), e.g., the power of the reference signal asreceived by the measuring device, averaged over the reportingmeasurement bandwidth. The metric may include a D2D Reference signalreceive quality (D2D-RSRQ), for example, the ratioN×D2D-RSRP/(D2D-RSSI), where N is the number of RBs of the D2D-RSSImeasurement bandwidth. The D2D-RSSI in this case may consist of thetotal received power averaged over the reference signal resources. Themetric may include any combination of these metrics, as well as othermeasurements, such as SINR, SNR, interference power, noise power, etc.

The measuring device may be configured to report channel qualityindication spanning a large bandwidth. In an example, the reportedbandwidth may be larger than the bandwidth allocated for the D2Dcommunication. The following examples may be used in any order orcombination.

The measuring device may be configured to report an independent qualitymetric for one or more set of subbands within the reference signalconfigured bandwidth. The measuring device may be configured to performtime-averaging and report the averaged measurement. For example, themeasuring device may be configured to measure the discovery signal or areference signal, which may be hopping in frequency with a knownpattern, and report the channel quality metric(s) for each of theconfigured set of subbands (or PRBs).

The measuring device may be configured to report for each configured setof subbands whether or not a specific quality metric threshold is met.For example, the measuring device may be configured with a qualitymetric and an associated minimum threshold. The measuring device mayreport for each configured subband whether or not the measured qualitymetric is above the threshold. For example, the measuring device may beconfigured to determine a quality metric based on a set of configuredD2D communications subbands. The measuring device may be configured toreport for each configured subband whether or not the measured qualitymetric is above the quality of the currently configured D2Dcommunication subbands. For example, the measuring device may beconfigured to report for each set of N_(D2D) subbands whether or not themeasured quality metric is above the quality of the set of currentlyconfigured N_(D2D) D2D communication subbands. The measuring device maybe configured to report the first N_(best) set of subbands for which thequality metric is above the threshold.

In yet another example, the measuring device may be configured to reportthe best N_(best) set of subbands with the associated metric (e.g., oneexample may be the case where N_(best)=1). The measuring device may beconfigured to not consider the set of subbands used for D2Dcommunications in the set to report. The eNB may determine analternative set of subbands for D2D communications.

The measuring device may be configured to report the worst N_(worst) setof subbands with the associated metric (e.g., one particular example isthe case where N_(worst)=1). The measuring device may be configured tonot consider the current set of subbands used for D2D communications inthe set to report. The eNB may determine an alternative set of subbandsfor D2D communications, for example, in avoiding the worst subbands ifpossible.

The measuring device may be configured to report a single metric in aspecific subband. The measuring device may determine the subband toreport for example based on eNB configuration (e.g., RRC, L2 or L1). Themeasuring device may be configured to not report for the currently usedset of subbands (e.g., for D2D communications). The transmitting devicemay be configured to determine the transmission parameters and may carrythis information to the receiving device. The receiving device maysignal information to the transmitting device. For example, thereceiving device may carry D2D control information or assignments (e.g.,DCI-like or UCI-like) for D2D communications or D2D-CI, to thetransmitting device. The PUCCH may be used to carry such assignments.The D2D-CI assignment may be protected in order to provide properreception and to not limit the data channel performance to the controlchannel performance. The encoding of the control channel information maybe described herein from the perspective of the WTRU transmitting theassignment. An inverse approach may be applied, for example, if the WTRUreceives the assignment. For example, the WTRU transmitting the D2D-CImay be configured to transmit the D2D-CI over the PUSCH or the PUSCHassociated to the D2D communication link (e.g., similar to the UCI inconventional LTE). In this approach, the WTRU transmitting the D2D-CImay multiplex the data with the control information, for example, usingan approach similar to UCI transmission over PUSCH. The controlinformation may also be transmitted over a MAC-CE, for example. When nodata is present, the WTRU may revert to one of the other approachesdisclosed herein or may transmit the control channel over the PUCCH. TheWTRU receiving the assignment may determine the presence of a D2D-CIassignment for example by decoding the header or blindly, by attemptingto decode a transport block of a specific size.

To reduce the D2D latency (e.g., overall latency), the D2D-CI may betransmitted separately from the data, e.g., to allow fast decoding. Thismay further allow a D2D assignment to be applied in the same subframe asit may be applied (or the subsequent subframe). This may be differentfrom an LTE system that may involve, e.g., four subframes to decode andapply the DCI information. The following examples may achieve reducedlatency.

A WTRU (e.g., to allow reduced latency) may be configured to transmitthe D2D-CI on one or more specific OFDM symbols. For example, asillustrated in FIG. 6, on an UL-like subframe 602, the controlinformation (D2D-CI) 604 may be transmitted in the OFDM symbol precedinga DM-RS symbol 606. The receiving WTRU may determine whether or not asymbol carries D2D-CI, for example, by blindly decoding the controlchannel. A small number of possible sizes may be configured for thecontrol channel, e.g., a concept utilized in the PDCCH.

In another example, to reduce latency, the WTRU may be configured tomultiplex the D2D-CI with one or both DM-RS symbols. For example, asillustrated in FIG. 7, on an UL-like subframe 702, the controlinformation (D2D-CI) may be multiplexed with the first DM-RS symbol andtransmitted on 704. The receiving WTRU may determine whether or not thissymbol also carries a D2D-CI for example by blindly decoding the controlchannel potentially contained in the DM-RS. This may be furthersimplified by configuring a small number of possible sizes for thecontrol channel, a concept utilized in the PDCCH. The receiving WTRU mayuse the decoded D2D-CI symbols (e.g., not to degrade the demodulationperformance due to the smaller number of DM-RS symbols) as additional(e.g., decision-directed) pilots after decoding of the D2D-CI (e.g.,conditioned on the CRC passing).

Although features and elements are described above in particularcombinations, one of ordinary skill in the art will appreciate that eachfeature or element can be used alone or in any combination with theother features and elements. In addition, the methods described hereinmay be implemented in a computer program, software, or firmwareincorporated in a computer-readable medium for execution by a computeror processor. Examples of computer-readable media include electronicsignals (transmitted over wired or wireless connections) andcomputer-readable storage media. Examples of computer-readable storagemedia include, but are not limited to, a read only memory (ROM), arandom access memory (RAM), a register, cache memory, semiconductormemory devices, magnetic media such as internal hard disks and removabledisks, magneto-optical media, and optical media such as CD-ROM disks,and digital versatile disks (DVDs). A processor in association withsoftware may be used to implement a radio frequency transceiver for usein a WTRU, UE, terminal, base station, RNC, or any host computer.

What is claimed is:
 1. A method of sending a reference signal (RS) from a transmitting wireless transmit/receive unit (WTRU) to a receiving WTRU, the method comprising: the transmitting WTRU determining an activation/deactivation status, wherein the activation/deactivation status is determined autonomously or based on an activation message received from a controlling device; on a condition the activation/deactivation status is activated, the transmitting WTRU transmitting a message to the receiving WTRU indicating an RS transmission activation; and the transmitting WTRU starting transmission of the RS to the receiving WTRU.
 2. The method of claim 1, on a condition the activation/deactivation status is determined autonomously, the activation/deactivation status is activated when one or more of following conditions are met: RS transmission is deactivated or the transmitting WTRU had an empty device-to-device (D2D) buffer and now the D2D buffer is not empty, the transmitting WTRU D2D buffer is above a threshold, the transmitting WTRU has changed its role from the receiving WTRU to the transmitting WTRU, D2D data link block error rate (BLER) is above a threshold value, or a timer has expired.
 3. The method of claim 1, on a condition the activation/deactivation status is received from the controlling device, the activation/deactivation status is received via a physical uplink control channel (PUCCH), a physical uplink shared channel (PUSCH) via a MAC control element (MAC-CE), a D2D PUCCH, or a D2D PUSCH via a MAC-CE.
 4. The method of claim 1, wherein the RS transmission activation is sent to an eNodeB or the receiving WTRU.
 5. The method of claim 4, on a condition the RS transmission activation is sent to the eNodeB, sending the RS transmission activation using PUCCH or PUSCH via a MAC-CE.
 6. The method of claim 4, on a condition the RS transmission activation is sent to the receiving WTRU, sending the RS transmission activation using D2D PUCCH or D2D PUSCH via a MAC-CE.
 7. The method of claim 1, wherein the RS is transmitted using a configured schedule.
 8. The method of claim 1 further comprising deactivating RS transmission, wherein the RS transmission is deactivated autonomously or based on a deactivation message received from the controlling device.
 9. The method of claim 1, on a condition the activation/deactivation status is determined autonomously, the activation/deactivation status is deactivated when one or more of following conditions are met: a transmitting WTRU D2D buffer is empty, the transmitting WTRU D2D buffer is below a threshold, or a D2D session is terminated.
 10. A transmitting wireless transmit/receive unit (WTRU) comprising: a processor configured to: determine an activation/deactivation status, wherein the activation/deactivation status is determined autonomously or based on an activation message received from a controlling device; on a condition the activation/deactivation status is activated, the transmitting WTRU transmitting a message to a receiving WTRU indicating a reference signal (RS) transmission activation; and the transmitting WTRU starting transmission of the RS to the receiving WTRU.
 11. The WTRU of claim 10, on a condition the activation/deactivation status is determined autonomously, the activation/deactivation status is activated when one or more of following conditions are met: RS transmission is deactivated or the transmitting WTRU had an empty D2D buffer and now the D2D buffer is not empty, the transmitting WTRU D2D buffer is above a threshold, the transmitting WTRU has changed its role from the receiving WTRU to the transmitting WTRU, D2D data link block error rate (BLER) is above a threshold value, or a timer has expired.
 12. The WTRU of claim 10, on a condition the activation/deactivation status is received from the controlling device, the activation/deactivation status is received via a physical uplink control channel (PUCCH), a physical uplink shared channel (PUSCH) via a MAC control element (MAC-CE), a D2D PUCCH, or a D2D PUSCH via a MAC-CE.
 13. The WTRU of claim 10, wherein the RS transmission activation is sent to an eNodeB or the receiving WTRU.
 14. The WTRU of claim 13, on a condition the RS transmission activation is sent to the eNodeB, the processor is further configured to send the RS transmission activation using a physical uplink control channel (PUCCH) or a physical uplink shared channel (PUSCH) via a MAC-CE.
 15. The WTRU of claim 13, on a condition the RS transmission activation is sent to the receiving WTRU, the processor is further configured to send the RS transmission activation using D2D PUCCH or D2D PUSCH via a MAC-CE.
 16. The WTRU of claim 10, wherein the RS is transmitted using a configured schedule.
 17. The WTRU of claim 10, the processor is further configured to deactivate RS transmission, wherein the RS transmission is deactivated autonomously or based on a deactivation message received from the controlling device.
 18. The WTRU of claim 10, on a condition the activation/deactivation status is determined autonomously, the activation/deactivation status is deactivated when one or more of following conditions are met: a transmitting WTRU device-to-device (D2D) buffer is empty, the transmitting WTRU D2D buffer is below a threshold, or a D2D session is terminated.
 19. A method of receiving a reference signal (RS) from a transmitting wireless transmit/receive unit (WTRU) to a receiving WTRU, the method comprising: the receiving WTRU sending a RS request to a controlling device; the receiving WTRU receiving, from the transmitting WTRU, a message indicating an RS transmission activation; the receiving WTRU monitoring an RS from the transmitting WTRU; and the receiving WTRU transmitting a channel state information (CSI) feedback to the transmitting WTRU.
 20. The method of claim 19, wherein the RS request to is sent to the controlling device via a physical uplink control channel (PUCCH), a physical uplink shared channel (PUSCH) via a MAC-CE, a D2D PUCCH, or a D2D PUSCH via a MAC-CE.
 21. The method of claim 19 further comprising stopping monitoring the RS from the transmitting WTRU.
 22. The method of claim 19 further comprising stopping transmitting of CSI feedback to the transmitting WTRU.
 23. The method of claim 19 further comprising sending an indication of RS deactivation to the controlling device.
 24. The method of claim 23, wherein the indication of RS deactivation is sent upon expiry of an inactivity timer when no data is received from the transmitting WTRU.
 25. A receiving wireless transmit/receive unit (WTRU) comprising: a processor configured to: sending a reference signal (RS) request to a controlling device; receive, from a transmitting WTRU, a message indicating a reference signal (RS) transmission activation; monitoring a reference signal (RS) from the transmitting WTRU; and sending a channel state information (CSI) feedback to the transmitting WTRU.
 26. The WTRU of claim 25, wherein the RS request to is sent to the controlling device via a physical uplink control channel (PUCCH), a physical uplink shared channel (PUSCH) via a MAC-CE, a D2D PUCCH, or a D2D PUSCH via a MAC-CE.
 27. The WTRU of claim 25, wherein the processor is further configured to stop monitoring the RS from the transmitting WTRU.
 28. The WTRU of claim 25, wherein the processor is further configured to stop transmitting of CSI feedback to the transmitting WTRU.
 29. The WTRU of claim 25, wherein the processor is further configured to send an indication of RS deactivation to the controlling device.
 30. The WTRU of claim 29, wherein the indication of RS deactivation is sent upon expiry of an inactivity timer when no data is received from the transmitting WTRU. 